sh-pfc: add macro to define pinmux without function

Used to define pinmux configurations where the pinmux function have no
representation in the configuration registers but instead solely depends
on a group selection.

Signed-off-by: Niklas Söderlund <niso@kth.se>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>